1. Field of the Invention
The present invention relates to a lubricating structure for outboard motors in which oil is supplied under pressure from an oil pump to related parts of a vertically installed engine.
2. Description of the Related Art
Conventionally, a V-type engine is known which has a pair of opposed cylinder blocks disposed in a V-shaped arrangement to form a so-called V-bank for compactness of the engine. In particular, an outboard motor is demanded to be light in weight and compact in size, so that an increasing number of V-type engines are applied to outboard motors. What is more, many V-type engines for outboard motors are vertically disposed with their crankshafts extending vertically.
In general, the engine of an outboard motor is constructed such that various parts inside the engine are lubricated by oil pumped up with an oil pump. To this end, the engine is formed therein with a main oil gallery through which lubricating oil passes. The main oil gallery vertically extends along the crankshaft of the engine between the two banks of a V-bank (i.e. in a central part of the engine in the transverse direction thereof), as disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H05-306633 (first prior-art lubricating structure for an outboard motor) or in Japanese Laid-Open Patent Publication (Kokai) No. H10-18827 (second prior-art lubricating structure for an outboard motor).
Particularly, an outboard motor has the maximum allowable width thereof limited depending on the width of a hull on which the outboard motor is installed. Particularly when two outboard motors are installed on a hull, for example, it is necessary to reduce the width of each outboard motor. For this reason, it has been considered advantageous for a vertically installed V-type engine of an outboard motor to have a reduced bank angle, and its design has been studied from this viewpoint.
However, when an engine is designed such that the bank angle of a V-bank is reduced (e.g. to 55 degrees or so) for reduction of the width of the engine, and a main oil gallery is formed in the central portion of the V-bank, an area where the main oil gallery can be formed is limited so as to avoid interference with a honing relief portion of a sleeve bore, for example. In addition, it is necessary to secure a sufficiently large inner diameter of oil passages for sufficient supply of lubricating oil from the main oil gallery to crank journals, which makes it difficult to form a main oil gallery having a sufficiently large cross-sectional area.
There has been also proposed a V-type engine having pistons thereof cooled by an oil jet. However, when oil passages for cooling the pistons are provided in such a conventional manner that they are in direct communication with the main oil gallery, a drop in oil pressure in the piston oil jet directly affects the main oil gallery to cause instability of oil pressure and the amount of oil to be supplied to a main journal.
In general, an outboard motor with a vertically installed engine, which is not limited to a V-type engine, has a lubricating structure in which oil stored in an oil pan disposed below the engine is pumped up with an oil pump, filtered by an oil filter, supplied under pressure to a main oil gallery, and then supplied as lubricating oil to related parts of the engine, such as crank journals, connecting rods, cylinders, and cylinder heads, through various oil passages formed within a cylinder block. An oil passage (supply passage) extending from the oil pump to the oil filter and oil passages (return passages) from the oil filter to the component parts of the engine are generally formed by machining, e.g. drilling the cylinder block such that cast through holes, which are linearly formed in the cylinder block, using a mold, communicate with each other.
On the other hand, an outboard motor has been known, which employs valve actuators for switching timing for opening and closing intake and exhaust valves between a high-speed mode and a low-speed mode. In the outboard motor, for example, oil pressure to be supplied to a variable valve timing mechanism provided at one end of a camshaft is switched by an oil control valve to thereby change the timing for opening and closing the intake and exhaust valves.
However, if oil returned through an oil filter is used both for driving the variable valve timing mechanism and lubricating cylinder heads, pressure variation in oil for lubricating the cylinder heads affects oil for driving the variable valve timing mechanism, which makes the operation of the variable valve timing mechanism unstable. To solve this problem, an engine for an outboard motor disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-342812 (third prior-art lubricating structure for an outboard motor) is equipped with a dedicated oil pump for driving a variable valve timing mechanism, in addition to an oil pump for lubrication, whereby the pressure of oil supplied to the variable valve timing mechanism is stabilized.
However, in the case of forming a supply passage to an oil filter and a return passage from the same in a cylinder block as in the above conventional lubricating structure for a general outboard motor, it is necessary to dispose the two passages such that they cannot interfere with cylinders, water jackets, chain transmission mechanisms, and so forth, and therefore the degree of freedom in laying out oil passages is strictly limited. Therefore, the cylinder block is apt to have an increased thickness, which hinders effective utilization of space. This is against the demand for compactness of outboard motors. That is, the outboard motor is desired to be compact in both height and width for the purpose of avoiding interference between a portion thereof located inside a hull and the hull itself when tilted up and maintaining excellent steerability, and due to limitation of the width thereof when two outboard motors are used for operation, it is significant to enhance the degree of freedom in laying out oil passages, for effective utilization of space.
Further, when two cast through holes are communicated with each other by machining, the oil passage formed thereby has a portion curved substantially at right angles, which increases fluid resistance, and makes contamination likely to occur owing to machining burrs. Furthermore, communication of the oil passage with defective cast portions (e.g. porosities within the passage) can cause oil leakage and lowering of oil pressure, thereby hindering smooth flow of oil.
On the other hand, also in the case where oil from the oil filter is used both for driving the variable valve timing mechanism and lubricating the cylinder heads, when the above third prior-art lubricating structure for an outboard motor is employed, it is necessary to additionally provide an oil pump, which complicates the construction of the outboard motor and increases manufacturing costs.